1Field of the Invention
The invention relates to an optical communications system, and more particularly to a system in which signals are transmitted in the form of coincidences between optical beams.
2. Discussion of Prior Art
Optical communications systems based on coincident photon pairs have been described previously, for instance by Hong, Friberg and Mandel in Applied Optics Vol. 24, No 22, Pages 3877-3882. The system they describe employs a non-linear crystal to produce simultaneous photon pairs by non-degenerate parametric downconversion of an input pump beam. These photon pairs form two beams of correlated photons, each beam including one photon of each pair. The beams are transmitted separately and in a receiver are detected separately. When a correlated photon pair is detected, one in each beam, a coincidence is recorded. A digital signal is transmitted by direct modulation of the input pump beam, that is by switching it on and off. The off periods in both beams are filled in using light which is spectrally similar to the downconverted photon pairs. The fill-in light is not, however, correlated, and therefore when it is detected in the receiver significant numbers of coincidences are not recorded. Thus periods of coincidences in the receiver correspond to binary one digits and periods of no coincidences (above noise) correspond to binary zero digits.
The system described has the advantage that the use of coincident detection allows discrimination against background noise to be achieved. Communication may therefore be achieved with relatively few photons and where the signal photons would otherwise be lost in the noise in each of the two beams.
The system is also relatively secure from interception due to the fill-in light. If only one beam is intercepted, and the light used to fill in is sufficiently well matched to the signal photons, then the signal cannot be decoded. However, matching the spectrum, intensity and statistical properties of the downconverted photons is difficult in practice. Consequently, detailed analysis of the properties of one beam would, in many cases enable the signal to be decoded. In addition, if both beams are intercepted the signal may be decoded by simple coincidence counting. The system is not therefore very secure.
A similar system employing time modulation in place of direct modulation has also been described. This time modulation system has two pulsed correlated photon beams. A digital signal is transmitted by time modulation of one of the two correlated beams. That is a variable delay is introduced into one of the pulsed beams. As with the direct modulation system described above, if one of the two beams is intercepted the signal cannot be decoded simply. However, careful analysis of statistics of time delays between pulses would enable some modulation to be detected. If the delays used are short compared to the period between pulses of the unmodulated beam, then the modulation detected by such analysis would be minimal. If both beams are intercepted, then as for the direct modulation system, simple coincidence counting will decode the signal. This system also, therefore, is not very secure.
Both of the prior art systems described above also suffer from the severe disadvantage of quadratic reduction in signal to noise as a function of loss in both channels. This puts very real constraints on the practical applications of the systems, particularly limiting the distances over which they may be used.